Norm of Somatosensory Evoked Potential
Results of the somatosensory evoked potential (SEP) are interpreted by a physician trained in neurophysiology.
Usage of Somatosensory Evoked Potential
Aids in the diagnosis of demyelinating diseases, including multiple sclerosis; neurodegenerative diseases, including adrenoleukodystrophy, adrenomyeloneuropathy, and Friedreich's ataxia; and spinal cord lesions. May help predict recovery prognosis in coma, especially nontraumatic coma.
Description of Somatosensory Evoked Potential
SEP testing uses peripheral electrical nerve stimulation to examine the conduction velocity of impulses through the somatosensory pathway along peripheral nerves to the cortex of the brain in a fashion similar to that of the electroencephalogram (EEG). The test uses sophisticated signal averaging to filter out the effect of other brain activity during testing. Of significance are conduction time for the SEP to occur after stimulation (latency) and the amplitude of the SEP waveform.
Professional Considerations of Somatosensory Evoked Potential
Consent form NOT required.
- Obtain EEG electrodes, an EEG machine, and electroconductive gel.
- Remove jewelry and metal objects from the client's head and limbs.
- Scalp electrodes are placed over the sensory cortex of the scalp on the side opposite that to be stimulated.
- Small painless electrical stimuli are administered to large sensory fibers in the median or posterior tibial nerves.
- The afferent volley is recorded as well as waves that reflect peripheral nerve trunk activity.
- Remove electrodes from the scalp and cleanse scalp of electroconductive gel.
Client and Family Teaching
- The hair should be clean, dry, and free of hair spray or other hair fixatives.
- Small, painless electrical stimuli are administered to peripheral nerves. The brain's response is recorded by means of scalp electrodes.
Factors That Affect Results
- The client must be able to lie motionless during the test.
- Results must be compared with the norms of the laboratory performing the test because different variations of the test will be performed, depending on the client's history and the purpose of the test.
- Complete lesion of the spinal cord results in no SEP recording when nerves distal to the lesion are stimulated.
- Lesions between the stimulated nerve and the thalamus increase the latency of the SEP.
- Lesions of the somatosensory cortex reduce the amplitude of the SEP wave.
- SEPs are a useful diagnostic tool for infants and children; however, growth and maturation of the nervous system complicate the technical application and interpretation of the results.
- SEP examines a restricted anatomic pathway and does not reflect general brainstem or cerebral function.
- This test is unaffected by general anesthesia, medications, and metabolic abnormalities.